The present invention relates to optical devices, and, more particularly, to light modulators of varying carrier densities in semiconductor materials.
Highly reflective mirrors made of alternating layers of non-absorbing materials are well known; see Jenkins and White, Fundamentals of Optics, ch 14 (McGraw Hill 1957), and FIG. 1A for a schematic perspective view of such a mirror and FIG. 1B for a graph of the reflectance. In such mirrors the two layer types have different optical constants ("n" is the index of refraction and "k" the attenuation so that .epsilon.=(n+ik).sup.2); and because there is a discontinuity in the optical constants at each layer interface, light which enters the mirror undergoes multiple reflections. If the optical thickness of each layer is chosen correctly (a quarter wavelength plus optional multiples of a half wavelength), the reflected rays will be in phase as illustrated in FIG. 1C and the mirror will have high reflectivity as illustrated in FIG. 1B. Narrow band reflectivity of 98% is routinely obtained in such multilayer structures.
To make such a multilayer mirror efficient, the optical absorption of each layer must be very small. Otherwise, significant optical absorption will take place within each layer and the subsequent multiple reflections within the mirror will further reduce the intensity of the internal light rays.
If the optical constants of the layers of such a multilayer mirror could be switched between absorbing and non-absorbing values, then a mirror of adjustable reflectivity (i.e. a light modulator) would result. But traditionally, the optical constants of a material can only be slightly adjusted by, for example, electric fields (the Pockels effect), which is insufficient to make a reasonable spatial light modulator. Thus it is a problem in the known multilayer mirrors to switch the optical constants of the layers.
Quantum well devices are known in various forms, heterostructure lasers being a good example. Quantum well heterostructure lasers rely on the discrete energy levels in the quantum wells to achieve high efficiency and typically consist of a few coupled quantum wells; see, generally, Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2d Ed 1981). High Electron Mobility Transistors (HEMTs) are another type of quantum well device and typically use only one half of a quantum well (a single heterojunction) but may include a stack of a few quantum wells. The HEMT properties arise from conduction parallel to the heterojunctions and in the quantum well conduction or valence subbands; the conduction carriers (electrons or holes) are isolated from their donors or acceptors and this isolation limits impurity scattering of the carriers. See, for example, T. Drummond et al. Electron Mobility in Single and Multiple Period Modulation-Doped (Al,Ga)As/GaAs Heterostructures, 53 J. Appl. Phys. 1023 (1982). Superlattices consist of many quantum wells so tightly coupled that the individual wells are not distinguishable, but rather the wells become analogous to atoms in a lattice. Consequently, superlattices behave more like new types of materials than as groups of coupled quantum wells; see, generally, L. Esaki et al, Superfine Structure of Semiconductors Grown by Molecular-Beam Epitaxy, CRC Critical Reviews in Solid State Sciences 195 (April 1976). Chemla et al, U.S. Pat. No. 4,525,687 and T. Wood et al, High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a p-i-n Diode Structure, 44 Appl. Phys. Lett. 16 (1984) describe a multiple quantum well device for light modulation: an applied electric field perturbs the exciton photon absorption resonances near the fundamental edge of direct gap semiconductors and provides the modulation; the use of quantum wells confines carriers and enhances the exciton binding energy. Further, the applied field modifies the envelope wave functions of carriers in the quantum wells and thus the confinement energies and the exciton binding energy. The net effect of the quantum wells is a pronounced absorption by exciton resonances, and these resonances have energies which are easily modifiable by an applied electric field. However, such resonance is extremely sharp and it is a problem to modulate a fairly broad band of light.
Resonant tunneling devices are the simplest quantum well devices that exhibit quantum confinement and coupling and were first investigated by L. Chang et al. 24 Appl. Phys. Lett. 593 (1974), who observed weak structure in the current-voltage characteristics of resonant tunneling diodes at low temperatures. More recently, Sollner et al. 43 Appl. Phys. Lett. 588 (1983), have observed large negative differential resistance in such devices (peak-to-valley ratios as large as six to one have been obtained), and Shewchuk et al, 46 Appl. Phys. Lett. 508 (1985) and M. Reed, to appear, have demonstrated room temperature resonant tunneling. However, resonant tunneling devices have little optical application and it is a problem to apply resonant tunneling to optical devices.